Shift problem

Got a 84 4x4 Ramcharger last week. Trans wont shift automaticly. Wont downshift when I came to a stop. Have to downshift manually and then manually up-shift into second and third. The shift itself although done manually feels fine. No slipping. Dont know when (or if) the trans was serviced. Fluid looks and smell O.K. Truck set up for several years prior to purchase. It will downshift upon excelleration from 3rd to second and then upshift to 3rd automaticly. To be honest, I really havent had time to look at or drive it very much. I'm thinkin its something simple like a stuck a valve. I assume it uses a detent cable. Somebody rebuilt the carb to git it running after being stored for so long and I havent checked the cable yet. Figured it wouldnt downshift upon excelleration if it wasnt hooked up. ?????????????

Any ideas???

Thanks.

Robert